AITKIN — The number of zebra mussels in Lake Mille Lacs has dropped this year for the first time since they were found in 2005.

Minnesota outdoors officials say a survey this week found about 1,070 zebra mussels per square foot in Mille Lacs, one of the state’s top fishing lakes. That’s down by about 200 from a year ago.

Tom Jones, a treaties manager in the Department of Natural Resources’ Aitkin office, said it’s typical for an invading species to ramp up in big numbers and then fall back a bit. Jones said in other places invasives have often dropped by as much as a third after initial expansion.

Zebra mussels are designated as a destructive invasive species. They filter plankton from water, making it clearer and causing more vegetation to grow deeper in lakes, potentially damaging the fish food chain. They can also attach themselves and clog motors and water intakes, and their sharp edges can be hazardous to swimmers.

Jones said that researchers will need more data next year to see if this year is the start of a decline or a one-time thing.